Not applicable.
The present invention relates to a process for the preparation of hydrocarbons from synthesis gas, (i.e., a mixture of carbon monoxide and hydrogen), typically labeled the Fischer-Tropsch process. Particularly, this invention relates to the use of supported catalysts containing boron and a Fischer-Tropsch catalytic metal (such as cobalt, cobalt/ruthenium, and the like) for the Fischer-Tropsch process.
Large quantities of methane, the main component of natural gas, are available in many areas of the world. Methane can be used as a starting material for the production of hydrocarbons. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step methane is reformed with water or partially oxidized with oxygen to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons. This second step, the preparation of hydrocarbons from synthesis gas is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s).
The Fischer-Tropsch reaction involves the catalytic hydrogenation of carbon monoxide to produce a variety of products ranging from methane to higher aliphatic alcohols. The process has been considered for the conversion of carbonaceous feedstock, e.g., coal or natural gas, to higher value liquid fuel or petrochemicals. The methanation reaction was first described in the early 1900""s, and the later work by Fischer and Tropsch dealing with higher hydrocarbon synthesis was described in the 1920""s. The first major commercial use of the Fischer-Tropsch process was in Germany during the 1930""s. More than 10,000 B/D (barrels per day) of products were manufactured with a cobalt based catalyst in a fixed-bed reactor. This work has been described by Fischer and Pichler in Ger. Pat. No. 731,295 issued Aug. 2, 1936, hereby incorporated herein by reference. Commercial practice of the Fischer-Tropsch process has continued from 1954 to the present day in South Africa in the SASOL plants. These plants use iron-based catalysts, and produce gasoline in relatively high-temperature fluid-bed reactors and wax in relatively low-temperature fixed-bed reactors.
The Fischer-Tropsch synthesis reactions are highly exothermic and reaction vessels must be designed for adequate heat exchange capacity. Because the feed streams to Fischer-Tropsch reaction vessels are gases while the product streams include liquids, the reaction vessels must have the ability to continuously produce and remove the desired range of liquid hydrocarbon products. Motivated by production of high-grade gasoline from natural gas, research on the possible use of the fluidized bed for Fischer-Tropsch synthesis was conducted in the United States in the mid-1940s. Based on laboratory results, Hydrocarbon Research, Inc. constructed a dense-phase fluidized bed reactor, the Hydrocol unit, at Carthage, Tex., using powdered iron as the catalyst. Due to disappointing levels of conversion, scale-up problems, and rising natural gas prices, operations at this plant were suspended in 1957. Research has continued, however, on developing Fischer-Tropsch reactors such as slurry-bubble columns, as disclosed in U.S. Pat. No. 5,348,982 issued Sep. 20, 1994, hereby incorporated herein by reference.
Catalysts for use in the Fischer-Tropsch synthesis usually contain a catalytically active metal of Groups 8, 9, 10 (in the New notation of the periodic table of the elements, which is followed throughout). In particular, iron, cobalt, nickel, and ruthenium, and combinations thereof, have been abundantly used as the catalytically active metals. Cobalt and ruthenium have been found to be particularly suitable for catalyzing a process in which synthesis gas is converted to primarily hydrocarbons having five or more carbon atoms (i.e., where the C5+ selectivity of the catalyst is high). However, due to its expense and rarity, ruthenium is typically used in combination with another of the catalytically active metals, such as cobalt. For example, U.S. Pat. No. 4,088,671, hereby incorporated herein by reference, discloses a process for the synthesis of higher hydrocarbons from the reaction of CO and hydrogen at low pressure in the contact presence of a catalyst comprising as the active ingredients a major amount of cobalt and a minor amount of ruthenium.
Additionally, the catalysts often contain a support or carrier material. Supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been refractory oxides (e.g., silica, alumina, titania, zirconia or mixtures thereof, such as silica-alumina). A support may be used to provide a high surface area for contact of the catalytically active metal with the syngas, to reduce the amount of catalytically active metal used, or to otherwise improve the performance or economics of catalysts and catalytic processes.
Additionally, Fischer-Tropsch catalysts often contain one or more promoters. For example, promoters that have been used for cobalt-ruthenium catalysts include thorium, lanthanum, magnesium, manganese, and rhenium. A promoter may have any of various desirable functions, such as improving activity, productivity, selectivity, lifetime, regenerability, or other properties of catalysts and catalytic processes.
There are significant differences in the molecular weight distributions of the hydrocarbon products from Fischer-Tropsch reaction systems. Product distribution or product selectivity depends heavily on the type and structure of the catalysts and on the reactor type and operating conditions. Accordingly, it is highly desirable to maximize the selectivity of the Fischer-Tropsch synthesis to the production of high-value liquid hydrocarbons, such as hydrocarbons with five or more carbon atoms per hydrocarbon chain.
Research is continuing on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream. High value hydrocarbons include those useful for further processing to yield gasoline, for example C5+ hydrocarbons, particularly C5-C10 hydrocarbons, and those useful for further processing to yield diesel fuel, for example C11+ hydrocarbons are, particularly C11-C20 hydrocarbons. A number of studies describe the behavior of iron, cobalt or ruthenium based catalysts in various reactor types, together with the development of catalyst compositions and preparations. For example, see the articles xe2x80x9cShort history and present trends of Fischer-Tropsch synthesis,xe2x80x9d by H. Schlutz, Applied Catalysis A 186, 3-12, 1999, and xe2x80x9cStatus and future opportunities for conversion of synthesis gas to liquid fuels, by G. Alex Mills, Fuel 73, 1243-1279, 1994, each hereby incorporated herein by reference in their entirety.
Notwithstanding the above teachings, it continues to be desirable to improve the activity and reduce the cost of Fischer-Tropsch catalysts and processes. In particular, there is still a great need to identify new promoted catalysts useful for Fischer-Tropsch synthesis, particularly catalysts that provide high C11+ hydrocarbon selectivities to maximize the value of the hydrocarbons produced and thus the process economics.
This invention provides a process and catalyst for producing hydrocarbons, and a method for preparing the catalyst. The process comprises contacting a feed stream comprising hydrogen and carbon monoxide with a catalyst in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising hydrocarbons.
In accordance with this invention, the catalyst used in the process comprises boron and a Fischer-Tropsch metal. The Fischer-Tropsch metal may include cobalt. Further, the Fischer-Tropsch metal additionally includes ruthenium or platinum.
This invention also includes a method for the preparation of a supported Fischer-Tropsch catalyst comprising supporting boron and cobalt and optionally ruthenium or platinum on a support material selected from the group including silica, titania, titania/alumina, zirconia, alumina, aluminum fluoride, and fluorided alumina.
This invention also provides a process for producing hydrocarbons, comprising contacting a feed stream comprising hydrogen and carbon monoxide with a supported catalyst in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising hydrocarbons. In accordance with this invention, the catalyst used in the process comprises boron and cobalt and optionally ruthenium or platinum, and one or more support materials selected from the group including silica, titania, titania/alumina, zirconia, alumina, aluminum fluoride, and fluorided aluminas.
The present catalyst contains a catalytically effective amount of a Fischer-Tropsch metal. The amount of metal present in the catalyst may vary widely. Typically, when the catalyst includes a support, the catalyst comprises from about 1 to 50% by weight (as the metal) of the total supported metal per total weight of catalytic metal and support, preferably from about 1 to 35% by weight, and more preferably from about 1 to 30% by weight. A Fischer-Tropsch metal may include an element selected from among a Group 8 element (e.g. Fe, Ru, and Os), a Group 9 element (e.g. Co, Rh, and Ir), a Group 10 element (e.g. Ni, Pd, and Pt), and combinations thereof. Preferably, the Fischer-Tropsch metal includes cobalt. More preferably, the Fischer-Tropsch metal further includes ruthenium or platinum. Ruthenium is preferably added to the catalyst in a concentration sufficient to provide a weight ratio of elemental ruthenium: elemental cobalt of from about 0.00005:1 to about 0.25:1, preferably from about 0.0005:1 to about 0.05:1, most preferably from about 0.0005:1 to 0.01:1 (dry basis). Platinum is preferably added to the catalyst in a concentration sufficient to provide a weight ratio of elemental platinum: elemental cobalt of from about 0.00001:1 to about 0.1:1, preferably from about 0.00005:1 to about 0.05:1, most preferably from about 0.0001:1 to 0.001:1 (dry basis).
We have found that higher selectivity and productivity catalysts are produced when boron is added to the catalyst. This is quite surprising because boron is typically considered to be a Fischer-Tropsch catalyst poisons. Productivites in batch testing can equal or exceed 500 g/hr/kg-cat. Further, the productivity increase with respect to the corresponding unpromoted catalyst can equal or exceed 20%. Likewise, the CO selectivity increase, and the methane selectivity decrease with respect to the corresponding unpromoted catalyst can each equal or exceed 20%. Additionally, and even more surprisingly, the catalysts of the present invention exhibit both improved conversion and improved stability, with long lifetime, relative to prior art Fischer-Tropsch catalysts. The amount of promoter is added to the catalyst in a concentration sufficient to provide a weight ratio of elemental promoter: elemental catalytic metal of from about 0.00005:1 to about 0.5:1, preferably, from about 0.0005:1 to about 0.01:1 (dry basis).
The active catalyst components used in this invention may be carried or supported on a support. Suitable supports include silica, titania, titania/alumina, zirconia, alumina, aluminum fluoride, and fluorided alumina, silica, titania, titania/alumina, silica/alumina, and the like, preferably alumina. Aluminum fluoride supports are defined as at least one aluminum fluoride (e.g., alpha-AlF3, beta-AlF3, delta-AlF3, eta-AlF3, gamma-AlF3, kappa-AlF3 and/or theta-AlF3). Fluorided alumina is defined as a composition comprising aluminum, oxygen, and fluorine. The fluoride content of the fluorided alumina can vary over a wide range, from about 0.001% to about 67.8% by weight. A preferred fluorided alumina contains from 0.001% to about 10% by weight fluorine. The remainder of the fluorided alumina component will include aluminum and oxygen. The composition may also contain a minor amount (compared to aluminum) of silicon, titanium, phosphorus, zirconium and/or magnesium.
The support material comprising fluorided aluminas and/or an aluminum fluoride may be prepared by a variety of methods. For example, U.S. Pat. Nos. 4,275,046 and 4,902,838 and 5,243,106 disclose the preparation of fluorided alumina by the reaction of alumina with a vaporizable fluorine-containing fluorinating compound. Suitable fluorinating compounds include HF, CCl3F, CCl2F2, CHClF2, CH3CHF2, CCl2FCClF2 and CHF3. U.S. Pat. No. 5,243,106 discloses the preparation of a high purity AlF3 from aluminum sec-butoxide and HF.
Phases of aluminum fluoride such as eta, beta, theta and kappa can be prepared as described in U.S. Pat. Nos. 5,393,509, 5,417,954, and 5,460,795.
Aluminas that have been treated with fluosilicic acid (H2SiF6) such as those described in European Patent Application No. EP 497,436 can also be used as a support. The disclosed support comprises from about 0.5 to about 10 weight percent of fluorine, from 0.5 to about 5 weight percent of silica and from about 85 to about 99 weight percent of alumina.
It will be understood that alternative supports are contemplated. A support may be selected according to desirable structural properties. Further a support may include any suitable composition.
Metals can be supported on aluminum fluoride or on fluorided alumina in a variety of ways. For example, U.S. Pat. No. 4,766,260 discloses the preparation of metals such as cobalt on a fluorided alumina support using impregnation techniques to support the metal. U.S. Pat. No. 5,559,069 discloses the preparation of a multiphase catalyst composition comprising various metal fluorides including cobalt fluoride homogeneously dispersed with aluminum fluoride. PCT Int. Publ. No. 97/19751 discloses the preparation of multiphase catalyst compositions comprising metallic ruthenium homogeneously dispersed with various metal fluorides including aluminum fluoride.
The catalysts of the preferred embodiments of the present invention may be prepared by any of the methods known to those skilled in the art. By way of illustration and not limitation, methods for preparing supported catalysts include impregnating the catalytically active compounds or precursors onto a support, extruding one or more catalytically active compounds or precursors together with support material to prepare catalyst extrudates, and/or precipitating the catalytically active compounds or precursors onto a support. Accordingly, a supported catalysts according to a preferred embodiment of the present invention may be used in the form of powders, particles, pellets, monoliths, honeycombs, packed beds, foams, and aerogels.
The most preferred method of preparation may vary among those skilled in the art, depending for example on the desired catalyst particle size. Those skilled in the art are able to select the most suitable method for a given set of requirements.
One method of preparing a supported metal catalyst is by incipient wetness impregnation of the support with an aqueous solution of a soluble metal salt such as nitrate, acetate, acetylacetonate or the like. Another method of preparing a supported metal catalyst is by a melt impregnation technique, which involves preparing the supported metal catalyst from a molten metal salt. One preferred method is to impregnate the support with a molten metal nitrate (e.g., Co(NO3)2.6H2O). Alternatively, the support can be impregnated with a solution of zero valent metal precursor. One preferred method is to impregnate the support with a solution of zero valent cobalt such as Co2(CO)8, Co4(CO)12 or the like in a suitable organic solvent (e.g., toluene). Suitable ruthenium compounds are the common water soluble ones, e.g., Ru(NH3)6Cl3 and Ru(III)nitrosylnitrate, and the common organic solvent, e.g. CH3CN, soluble ones, e.g. Ru(II)2,4-pentanedionate. Suitable platinum compounds include platinum(II)acetylacetonate. Suitable boron compounds are the common water soluble ones, e.g. boria (B2O3).
The most preferred sequence of addition of elements to a support may vary among those skilled in the art. For example, it is contemplated that the Fischer-Tropsch metal and boron may be added to a support in the same mixture. Alternatively, the Fischer-Tropsch metal and the boron may be added in separate steps. Further, each element may be added in any one or more of the steps of a multiple impregnation. Still further, a supported catalyst according to a preferred embodiment of the present invention may include co-dispersed Fischer-Tropsch metal and boron. Alternatively, a supported catalyst according to a preferred embodiment of the present invention may include a layer containing a Fischer-Tropsch metal and a layer containing boron. This is particularly surprising as boron has been considered to be act as a Fischer-Tropsch poison by covering the Fischer-Tropsch metal.
It will be understood that the promoter is preferably added as a component of the materials loaded on the support, and is thus preferably distinct from a modifier of the support itself.
The impregnated support is dried and reduced with hydrogen or a hydrogen containing gas. The hydrogen reduction step may not be necessary if the catalyst is prepared with zero valent cobalt. In another preferred method, the impregnated support is dried, oxidized with air or oxygen and reduced in the presence of hydrogen.
Typically, at least a portion of the metal(s) of the catalytic metal component (a) of the catalysts of the present invention is present in a reduced state (i.e., in the metallic state). Therefore, it is normally advantageous to activate the catalyst prior to use by a reduction treatment, in the presence of hydrogen at an elevated temperature. Typically, the catalyst is treated with hydrogen at a temperature in the range of from about 75xc2x0 C. to about 500xc2x0 C., for about 0.5 to about 24 hours at a pressure of about 1 to about 75 atm. Pure hydrogen may be used in the reduction treatment, as may a mixture of hydrogen and an inert gas such as nitrogen, or a mixture of hydrogen and other gases as are known in the art, such as carbon monoxide and carbon dioxide. Reduction with pure hydrogen and reduction with a mixture of hydrogen and carbon monoxide are preferred. The amount of hydrogen may range from about 1% to about 100% by volume.
The catalysts of the preferred embodiments of the present invention are preferably used in a catalytic process for production of hydrocarbons, most preferably the Fischer-Tropsch process. The feed gases charged to the process of the preferred embodiment of the present invention comprise hydrogen, or a hydrogen source, and carbon monoxide. H2/CO mixtures suitable as a feedstock for conversion to hydrocarbons according to the process of this invention can be obtained from light hydrocarbons such as methane by means of steam reforming, partial oxidation, or other processes known in the art. Preferably the hydrogen is provided by free hydrogen, although some Fischer-Tropsch catalysts have sufficient water gas shift activity to convert some water to hydrogen for use in the Fischer-Tropsch process. It is preferred that the molar ratio of hydrogen to carbon monoxide in the feed be greater than 0.5:1 (e.g., from about 0.67 to 2.5). Preferably, the feed gas stream contains hydrogen and carbon monoxide in a molar ratio of about 2:1. The feed gas may also contain carbon dioxide. The feed gas stream should contain a low concentration of compounds or elements that have a deleterious effect on the catalyst, such as poisons. For example, the feed gas may need to be pre-treated to ensure that it contains low concentrations of sulfur or nitrogen compounds such as hydrogen sulfide, ammonia and carbonyl sulfides.
The feed gas is contacted with the catalyst in a reaction zone. Mechanical arrangements of conventional design may be employed as the reaction zone including, for example, fixed bed, fluidized bed, slurry phase, slurry bubble column, reactive distillation column, or ebullating bed reactors, among others, may be used. Accordingly, the size and physical form of the catalyst particles may vary depending on the reactor in which they are to be used.
The Fischer-Tropsch process is typically run in a continuous mode. In this mode, the gas hourly space velocity through the reaction zone typically may range from about 100 volumes/hour/volume catalyst (v/hr/v) to about 10,000 v/hr/v, preferably from about 1000 v/hr/v to about 8,000 v/hr/v. The reaction zone temperature is typically in the range from about 160xc2x0 C. to about 300xc2x0 C. Preferably, the reaction zone is operated at conversion promoting conditions at temperatures from about 190xc2x0 C. to about 260xc2x0 C. The reaction zone pressure is typically in the range of about 80 psig (653 kPa) to about 1000 psig (6994 kPa), preferably, from 80 psig (653 kPa) to about 600 psig (4237 kPa), and still more preferably, from about 140 psig (1066 kPa) to about 450 psig (2858 kPa).
The products resulting from the process will have a great range of molecular weights. Typically, the carbon number range of the product hydrocarbons will start at methane and continue to the limits observable by modern analysis, about 50 to 100 carbons per molecule. The process is particularly useful for making hydrocarbons having five or more carbon atoms especially when the above-referenced preferred space velocity, temperature and pressure ranges are employed.
The wide range of hydrocarbons produced in the reaction zone will typically afford liquid phase products at the reaction zone operating conditions. Therefore the effluent stream of the reaction zone will often be a mixed phase stream including liquid and vapor phase products. The effluent stream of the reaction zone may be cooled to effect the condensation of additional amounts of hydrocarbons and passed into a vapor-liquid separation zone separating the liquid and vapor phase products. The vapor phase material may be passed into a second stage of cooling for recovery of additional hydrocarbons. The liquid phase material from the initial vapor-liquid separation zone together with any liquid from a subsequent separation zone may be fed into a fractionation column. Typically, a stripping column is employed first to remove light hydrocarbons such as propane and butane. The remaining hydrocarbons may be passed into a fractionation column where they are separated by boiling point range into products such as naphtha, kerosene and fuel oils. Hydrocarbons recovered from the reaction zone and having a boiling point above that of the desired products may be passed into conventional processing equipment such as a hydrocracking zone in order to reduce their molecular weight. The gas phase recovered from the reactor zone effluent stream after hydrocarbon recovery may be partially recycled if it contains a sufficient quantity of hydrogen and/or carbon monoxide.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present invention to its fullest extent. The following embodiments are to be construed as illustrative, and not as constraining the scope of the present invention in any way whatsoever. For example, it will be understood that while some continuous testing is described, a process for producing hydrocarbons may alternatively be operated in batch mode.